join Linq

List<Publisher> Publishers = new List<Publisher>();
            Publisher publish1 = new Publisher();
            publish1.Code = "01";
            publish1.Name = "北京出版社1";
            Publishers.Add(publish1);

            Publisher publish2 = new Publisher();
            publish2.Code = "02";
            publish2.Name = "北京出版社2";
            Publishers.Add(publish2);

            List<Book> Books = new List<Book>();
            Book boo1 = new Book();
            boo1.Publisher = new Publisher() { Name = "北京出版社1",Code="01" };
            boo1.Title = "书名1";
            boo1.PublisherCode = "01";
            Books.Add(boo1);

            Book boo2 = new Book();
            boo2.Publisher = new Publisher() { Name = "北京出版社2",Code="02" };
            boo2.Title = "书名2";
            boo2.PublisherCode = "02";
            Books.Add(boo2);

            Book boo3 = new Book();
            boo3.Publisher = new Publisher() { Name = "北京出版社2",Code="02" };
            boo3.PublisherCode = "02";
            boo3.Title = "书名3";
            Books.Add(boo3);


            //使用组连接
            var GroupQuery = (from publisher in Publishers
                            join book in Books on publisher equals book.Publisher 
                            into publisherBooks
                            select new
                            {
                                PublisherName = publisher.Name,
                                Books = publisherBooks
                            }).ToList();

             //使用Group
            var QueryByGroup = (from book in Books
                        group book by book.Publisher 
                        into grouping
                        select new
                        {
                            PublisherName = grouping.Key.Name,
                            Books = grouping.ToList()
                        }).ToList();

            var joinQuery = from publisher in Publishers
                            join book in Books
                                on publisher equals book.Publisher


                            select new
                            {
                                PublisherName = publisher.Name,
                                BookName = book.Title
                            };
            var tem = joinQuery.ToList();

            //left join
            var joinQuery2 = (from publisher in Publishers
                              join book in Books on publisher.Code equals book.PublisherCode 
                              into publisherBooks
                              from publisherBook in publisherBooks.DefaultIfEmpty()
                            select new
                            {
                                PublisherName = publisher.Name,
                                BookName = (publisherBook == default(Book)) ? "no book" : publisherBook.Title
                            }).ToList();

            //cross join
            var crossJoinQuery = (from publisher in Publishers
                                 from book in Books
                                 select new
                                 {
                                     PublisherName = publisher.Name,
                                     BookName = book.Title
                                 }).ToList();

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